Conventional plasma sputter etching methods include DC (direct-current) diode sputtering, capacitively-coupled radio-frequency (RF) sputtering and downstream plasma sputtering. The first two methods require relatively high gas pressure (1 to 50 mTorr) and are unable to sustain a plasma discharge at low electrode voltages (below ˜300 volts). These characteristics limit the control of ion energies at the etched surface, particularly in the low energy regime, which may be important for low-damage etching of sensitive structures. The downstream plasma etching method uses a remote plasma source or plasma generator that, depending upon the characteristics of the plasma source, may allow lower pressure operation and may provide low-energy ion fluxes to the etched substrate. In most downstream plasma etching, a capacitively-coupled RF bias must still be applied to the substrate to accelerate ions to the substrate because the ion energies provided by the source would otherwise be too low to accomplish etching. Since the remote plasma source itself is often an inductively-coupled RF plasma generator, downstream plasma etching tends to be complex and expensive. Also, in common with simple capacitively-coupled RF sputtering, the capacitive coupling of RF power through the substrate to the plasma in downstream plasma etching has at least three limitations. (a) The angle of ion impact at the etched surface is constrained to normal incidence, since a thin plasma sheath at the substrate surface accelerates the ions toward the substrate. (b) The kinetic energy of the ions varies all the way from zero (as necessary for the substrate surface to become positively biased and receive a flux of electrons to neutralize positive surface charge build-up due to ion impact, on insulating substrates) to a kinetic energy characteristic of approximately the peak negative voltage of the RF waveform applied. (c) The substrate, being capacitively coupled, will self-adjust its electric potential (will “float”) such that equal numbers, on average, of ions and electrons will reach the surface of the substrate, without regard to the detailed spatial structure of conductors and insulators on the surface of the substrate; if, for example, over neutralization by excess electron flux is desired, this is very difficult to arrange.
To overcome these limitations of the present plasma etching technology, it would be desirable to provide a remote plasma source, as is advantageous in downstream plasma etching, but provide a source which is cheaper and simpler. In addition, it would be advantageous for controlling ion incidence angle on the substrate if the remote plasma source could also accelerate ions toward the substrate region and that the substrate holder be variable in angle with respect to the accelerated ion flux. Furthermore, when it is advantageous to bias the substrate to further accelerate ions, it would be preferable to impose more controlled, deterministic and constant-valued negative voltages, so that the ion kinetic energy during etching is more nearly mono-energetic. Likewise, when it is advantageous to bias the substrate to attract electrons, it would be preferable to impose more controlled, deterministic and constant-valued positive voltages, so that variable amounts of electrons can controllably be delivered to the surface of the substrate. Moreover, it would be highly desirable to provide means for independently controlling the ion currents and the electron currents at the substrate. It would be beneficial to provide better control of the etching process without adding complex circuitry to the apparatus arrangement.
The present invention is directed to these objectives.